Erdogan and Putin announce de-militarised zone between opposition and Syrian government forces in last rebel bastion.

The leaders of Russia and Turkey have agreed to establish a de-militarised zone in Syria's Idlib province, in a move that ostensibly puts on hold a threatened all-out assault by government forces on Syria's last rebel bastion.

The announcement was made on Monday during a press conference in the Black Sea resort of Sochi, where Russian President Vladimir Putin hosted his Turkish counterpart, Recep Tayyip Erdogan.

Speaking alongside Erdogan, Putin said the 15-20km-wide zone would be established by October 15.

This would entail a "withdrawal of all radical fighters" from Idlib, including the al-Nusra Front, Putin said, referring to Hay'et Tahrir al-Sham (HTS), which is dominated by a rebel faction previously known as al-Nusra Front before renouncing its ties to al-Qaeda.

Putin added that heavy weapons would be withdrawn from all opposition forces by October 10 - an approach supported by the government of Syrian President Bashar al-Assad.

By the end of 2018, transportation routes linking Syria's key port of Latakia with major cities Aleppo and Hama must also be restored, added the Russian president, a major Assad ally.

Describing the agreement as a "serious result", Putin said that "Russia and Turkey have confirmed their determination to counter terrorism in Syria in all its forms".

For his part, Erdogan said both his country and Russia would carry out coordinated patrols in the de-militarised zone, and reiterated that the biggest threat to Turkey was the Syrian Kurdish People's Protection Units (YPG), who control swaths of territory in northeast Syria.

"We decided on the establishment of a region that is cleaned of weapons between the areas which are under the control of the opposition and the regime," said Erdogan, whose administration backs certain rebel groups in Idlib.

"In return, we will ensure that radical groups, which we will designate together with Russia, won't be active in the relevant area," he added.

"We will prevent a humanitarian tragedy which could happen as a result of military action."

The United Nations has warned that a large-scale offensive on the northwestern province, home to three million people, would result in a "bloodbath" and lead to the "worst humanitarian catastrophe in the 21st century".

Following the press conference, Russian Defence Minister Sergei Shoigu said the agreement between Putin and Erdogan meant that no military action would be taken against Idlib, according to Russian news agencies.

Turkey's 'huge leverage'

The Syrian government had recently announced plans to launch a major military offensive on Idlib province, long controlled by various armed opposition groups, after managing to claw back swaths of rebel-held territory.

Since the beginning of September, dozens of people have been killed and wounded in air raids and attacks by the Syrian government and allied Russian fighter jets, according to activists on the ground.

Russia and Iran, who back the Syrian government, want to eliminate what they call "terrorist groups" in the province, which borders Turkey.

Ankara wants a stable ceasefire in the region to disarm these groups while keeping the peace in Idlib.

Turkish officials had repeatedly warned Russia and the Syrian government against attacking Idlib, saying it would lead to another massive wave of refugees heading towards Turkey.

Over the past week, Turkey has deployed reinforcements and expanded defensive structures at about a dozen observation points across opposition-held territories in Idlib, western Aleppo, and northern Hama provinces.

The outposts were established after a "de-escalation" agreement was reached between Turkey, Russia, and Iran in July 2017.

Marwan Kabalan, director of the Arab Center for Research and Policy Studies, said that following the deployment of troops, Turkey seemed to have "the right forces on the ground" in order to enforce Monday's agreement.

"The Turks seem to be very confident that they can actually implement this because they have a huge leverage right now on the 'radical' groups in Idlib," he said in Qatar's capital, Doha.

"Otherwise, there will be a military retaliation by the Russians - what the Turks are doing right now is providing some protection cover for these groups from any Russian attack. So these groups now, in my opinion, must bow to the Turkish pressure and try to cooperate."

On the other hand, Kabalan noted that a major military offensive "would had been very costly" for Russia.

"Firstly, they don't have enough manpower on the ground to start the operation," he said.

"Secondly, the agreement shows that Turkey has become a very important partner for Russia in Syria, and the Russians didn't want to sacrifice their partnership with Turkey in order to go and fight the opposition in Idlib.

"And thirdly, the Russians are more attentive right now to the United States' position on Idlib, which in recent days became even stronger against any military operation."

On Friday, thousands took to the streets across Idlib, a densely populated province where about half of the population are internally displaced persons, to protest against the threatened offensive.

Kabalan said the agreement signalled that when it comes to Idlib's fate, "it's only Putin and Erdogan, not even Iran" that are calling the shots, adding that both leaders "need each other right now in Syria ... for any political solution".